1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to a recording control technology in an image forming apparatus which forms images by means of dots formed by ejecting droplets onto a recording medium.
2. Description of the Related Art
Inkjet recording apparatuses (inkjet printers) having an inkjet head (ink ejection head) in which a plurality of nozzles are arranged, are known as image forming apparatuses. An inkjet recording apparatus of this kind forms images by forming dots on a recording medium, by ejecting ink as droplets from nozzles, while causing the inkjet head and the recording medium to move relatively to each other.
Various methods are known conventionally as ink ejection methods for an inkjet recording apparatus of this kind. For example, one known method is a piezoelectric method, where the volume of a pressure chamber (ink chamber) is changed by causing a vibration plate forming a portion of the pressure chamber to deform due to deformation of a piezoelectric element (piezoelectric actuator), ink being introduced into the pressure chamber from an ink supply passage when the volume is increased, and the ink inside the pressure chamber being ejected as a droplet from the nozzle when the volume of the pressure chamber is reduced. Another known method is a thermal inkjet method where ink is heated to generate a bubble in the ink, and ink is then ejected by means of the expansive energy created as the bubble grows.
In an inkjet recording apparatus, one image is represented by combining dots formed by ink ejected from the nozzles. High image quality can be achieved by making the dots small in size, increasing the density of the dots and thereby using a large number of pixels per image.
However, if the density of the dots is increased, then ink droplets (dots) which are adjacent or overlapping on the recording medium may become smeared or their colors may become mixed, and smearing or color mixing of this kind will cause the image quality to deteriorate.
Therefore, conventionally, various methods have been proposed in order to prevent smearing or color mixing of ink. For example, Japanese Patent Application Publication No. 5-4342 discloses a recording apparatus in which a recording liquid having electrorheological properties is formed into droplets by a recording head and caused to adhere to an intermediate transfer medium having an electric field created on the surface thereof, thereby raising the viscosity of the droplets on the transfer medium, and the recording liquid is then transferred in this state of increased viscosity onto a transfer receiving medium, thereby preventing excessive spreading or color mixing caused by the recording head and hence making it possible to achieve high-quality printing. However, in the recording apparatus described in Japanese Patent Application Publication No. 5-4342, time is required for the recording droplets to dry on the intermediate transfer medium, and this leads to a decline in recording speed. Furthermore, if the image is transferred in a state where an electric field is applied, then it is not possible to control smearing after transfer.
Japanese Patent Application Publication No. 5-4343 discloses a recording apparatus in which a recording liquid having electrorheological properties is formed into droplets by a recording head and caused to adhere to a transfer medium formed with an electric field, whereby the viscosity or yield value of the adhering droplets of recording liquid is increased. Therefore, blurring, smearing or color mixing of the recorded dots is prevented and high-quality printing can be achieved.
However, in the recording apparatus described in Japanese Patent Application Publication No. 5-4343, even if it is possible to restrict the rate of smearing of the recording dots by applying an electric field, smearing of the recording dots still continues for a long period of time after the electric field is removed.
Both of the aforementioned cases describe the possibility of preventing smearing of recording dots by applying an electric field to droplets of recording liquid having electrorheological properties; however, they do not disclose a method for controlling the permeation velocity of the recording droplets, at which the recording droplets permeate into the recording medium, to an optimum velocity, and neither do they give any explanation of the relationship between the permeation of the recording dots and smearing. Hence, there is a problem in that smearing of the recording dots, and the like, cannot be controlled optimally in accordance with the type of recording medium and recording liquid.